[BUILD] fix a minor build warning on AIX

AIX wants string.h in signal.c (and is right to do so) :

gcc -Iinclude -Wall  -O2 -g       -DTPROXY -DENABLE_POLL  -DCONFIG_HAPROXY_VERSION=\"1.3.18\" -DCONFIG_HAPROXY_DATE=\"2009/05/10\" -c -o src/signal.o src/signal.c
src/signal.c: In function 'signal_init':
src/signal.c:32: warning: implicit declaration of function 'memset'
src/signal.c:32: warning: incompatible implicit declaration of built-in function 'memset'
(cherry picked from commit be8c736cca)

src/signal.c

@@ -11,6 +11,8 @@
  */
 
 #include <signal.h>
+#include <string.h>
+
 #include <proto/signal.h>
 #include <proto/log.h>